Selecting the correct material is critical for footwear structural integrity. You should prioritize high-carbon spring steel (e.g., 50#, 65Mn, or 70#) to ensure the shank provides necessary longitudinal support without permanent deformation. The hardness level should typically fall between 42-52 HRC (Rockwell Scale); if it is too soft, the shoe will lose its shape, and if it is too brittle, the shank may snap under pressure. Always request a Heat Treatment Certification from the supplier to verify the tempering process.